The Big Bang theory is the prevailing cosmological model for the early development of the universe. According to the theory, the Big Bang occurred approximately 13.798 ± 0.037 billion years ago, which is thus considered the age of the universe.At this time, the universe was in an extremely hot and dense state and began expanding rapidly. After the initial expansion, the universe cooled sufficiently to allow energy to be converted into various subatomic particles, including protons, neutrons, and electrons. Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed. The majority of atoms that were produced by the Big Bang are hydrogen, along with helium and traces of lithium. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae.

The Big Bang is the scientific theory that is most consistent with observations of the past and present states of the universe, and it is widely accepted within the scientific community. It offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background, large scale structure, and the Hubble diagram.

The core ideas of the Big Bang - the expansion, the early hot state, the formation of light elements, and the formation of galaxiesÑare derived from these and other observations. As the distance between galaxies increases today, in the past galaxies were closer together. The consequence of this is that the characteristics of the universe can be calculated in detail back in time to extreme densities and temperatures,while large particle accelerators replicate such conditions, resulting in confirmation and refinement of the details of the Big Bang model. On the other hand, these accelerators can only probe so far into high energy regimes, and astronomers are prevented from seeing the absolute earliest moments in the universe by various cosmological horizons. The earliest instant of the Big Bang expansion is still an area of open investigation. The Big Bang theory does not provide any explanation for the initial conditions of the universe; rather, it describes and explains the general evolution of the universe going forward from that point on.

Georges Lema”tre first proposed what became the Big Bang theory in what he called his "hypothesis of the primeval atom". Over time, scientists built on his initial ideas to form the modern synthesis. The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions such as homogeneity and isotropy of space.

The governing equations were first formulated by Alexander Friedmann and similar solutions were worked on by Willem de Sitter. In 1929, Edwin Hubble discovered that the distances to far away galaxies were strongly correlated with their redshifts - an idea originally suggested by Lema”tre in 1927. Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity, regardless of direction. Assuming that we are not at the center of a giant explosion, the only remaining interpretation is that all observable regions of the universe are receding from each other.

While the scientific community was once divided between supporters of two different expanding universe theories - the Big Bang and the Steady State theory, observational confirmation of the Big Bang scenario came with the discovery of the cosmic microwave background radiation in 1964, and later when its spectrum (i.e., the amount of radiation measured at each wavelength) was found to match that of thermal radiation from a black body. Since then, astrophysicists have incorporated observational and theoretical additions into the Big Bang model, and its parametrization as the Lambda-CDM model serves as the framework for current investigations of theoretical cosmology. Read more ... Wikipedia

On March 17, 2014, the New York Times reported that astronomers at the Harvard-Smithsonian Center for Astrophysics detected gravitational waves, providing strong evidence for inflation and the Big Bang.

In the News ...

Is the universe ringing like a crystal glass? PhysOrg - June 27, 2015
Many know the phrase "the big bang theory." There's even a top television comedy series with that as its title. According to scientists, the universe began with the "big bang" and expanded to the size it is today. Yet, the gravity of all of this matter, stars, gas, galaxies, and mysterious dark matter, tries to pull the universe back together, slowing down the expansion. Now, two physicists at The University of Southern Mississippi, Lawrence Mead and Harry Ringermacher, have discovered that the universe might not only be expanding, but also oscillating or "ringing" at the same time.

Major Big Bang Discovery Brings 'Theory of Everything' a Bit Closer to Reality Live Science - March 21, 2014
The discovery that the universe really did expand at many times the speed of light immediately after the Big Bang should bring physicists slightly closer to their ultimate goal - the long-sought "Theory of Everything." The bottom part of this illustration shows the scale of the universe versus time. Specific events are shown such as the formation of neutral Hydrogen at 380 000 years after the big bang. Prior to this time, the constant interaction between matter (electrons) and light (photons) made the universe opaque. After this time, the photons we now call the CMB started streaming freely.

That Signal From the Beginning of Time Could Redefine Our Universe Wired - March 18, 2014
The physics world was on fire yesterday after an announcement that astronomers had detected a signal from the beginning of time. This is exactly as cool as it sounds. Maybe even cooler. And it might lead to us learning further crazy things about our universe. Besides coming as a shock to most of the community, the discovery once again proved that we don't quite know many things about our universe. Ordinarily sober-minded scientists went to hyperbolic lengths to describe just how significant the results were. Depending on who you ask, they were as important as finding the Higgs boson, directly detecting dark matter, or discovering life on other planets.

Cosmic Microwave Map Swirls Indicate Inflation NASA - March 17, 2014
Did the universe undergo an early epoch of extremely rapid expansion? Such an inflationary epoch has been postulated to explain several puzzling cosmic attributes such as why our universe looks similar in opposite directions. Yesterday, results were released showing an expected signal of unexpected strength, bolstering a prediction of inflation that specific patterns of polarization should exist in cosmic microwave background radiation -- light emitted 13.8 billion years ago as the universe first became transparent. Called B-mode polarizations, these early swirling patterns can be directly attributed to squeeze and stretch effects that gravitational radiation has on photon-emitting electrons. The surprising results were discovered in data from the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) microwave observatory near the South Pole. BICEP2 is the building-mounted dish pictured above on the left. Note how the black polarization vectors appear to swirl around the colored temperature peaks on the inset microwave sky map. Although statistically compelling, the conclusions will likely remain controversial while confirmation attempts are made with independent observations.

Cosmic inflation: Spectacular discovery hailed BBC - March 17, 2014
Scientists say they have extraordinary new evidence to support a Big Bang Theory for the origin of the Universe. Researchers believe they have found the signal left in the sky by the super-rapid expansion of space that must have occurred just fractions of a second after everything came into being. It takes the form of a distinctive twist in the oldest light detectable with telescopes. The breakthrough was announced by an American team working on a project known as BICEP2.

String Theorists Simulate the Big Bang Live Science - December 13, 2011
Japanese physicists have created a string theory model that simulates the birth of the universe. In their model, the Big Bang was a "symmetry-breaking event" - a fluctuation that caused three spatial dimensions to break free from the other six dimensions of string theory, then rapidly unfurl to produce our universe's observed 3D structure. String theory - a proposed "theory of everything" that unites quantum mechanics and general relativity together in one complete picture - models elementary particles as oscillating lines ("strings") rather than dimensionless points. In order for the math to work, string theory requires that there be 10 dimensions: nine of space and one of time. Our universe only appears to have three spatial dimensions, string theorists say, because the other six are curled up in undetectably tiny bundles called Calabi-Yau manifolds, which are a minuscule 10^-33 centimeters across.

String theory researchers simulate big-bang on supercomputer PhysOrg - December 14, 2011
A trio of Japanese physicists have applied a reformulation of string theory, called IIB, whereby matrices are used to describe the properties of the physical universe, on a supercomputer, to effectively show that the universe spontaneously ballooned in three directions, leaving the other six dimensions tightly wrapped, as string theory has predicted all along.

Could the Big Bang have been a quick conversion of antimatter into matter? PhysOrg - July 19, 2011
Suppose at some point the universe ceases to expand, and instead begins collapsing in on itself (as in the ÒBig CrunchÓ scenario), and eventually becomes a supermassive black hole. The black holeÕs extreme mass produces an extremely strong gravitational field. Through a gravitational version of the so-called Schwinger mechanism, this gravitational field converts virtual particle-antiparticle pairs from the surrounding quantum vacuum into real particle-antiparticle pairs. If the black hole is made from matter (antimatter), it could violently repel billions and billions of antiparticles (particles) out into space in a fraction of a second, creating an ejection event that would look quite similar to a Big Bang.

Matter Melts in Superhot Particle Collisions Live Science - June 23, 2011
By creating a soup of subatomic particles similar to what the Big Bang produced, scientists have discovered the temperature boundary where ordinary matter dissolves. Normal atoms will be converted into another state of matter - a plasma of quarks and gluons - at a temperature about 125,000 times hotter than the center of the sun, physicists said after smashing the nuclei of gold atoms together and measuring the results. While this extreme state of matter is far from anything that occurs naturally on Earth, scientists think the whole universe consisted of a similar soup for a few microseconds after the Big Bang about 13.7 billion years ago.

Cosmos may show echoes of events before Big Bang BBC - November 27, 2010
Evidence of events that happened before the Big Bang can be seen in the glow of microwave radiation that fills the Universe, scientists have asserted. Renowned cosmologist Roger Penrose said that analysis of this cosmic microwave background showed echoes of previous Big Bang-like events. The events appear as "rings" around galaxy clusters in which the variation in the background is unusually low.

Planck telescope beams back first images of the fall out after the Big Bang Telegraph.co.uk - September 17, 2009
The telescope was sent 0.9 million miles (1.5 million km) into space by the European Space Agency earlier this year to look at the age, contents and evolution of the cosmos. Now the first images have come back showing strips of ancient light across the sky.
It will take two years for scientists to analyze the images and begin to interpret what it tells us about the beginning of the universe.
The telescope is looking at the heat left by the Big Bang millions of years ago known as Cosmic Microwave Background radiation (CMB).

Cosmic 'treasure trove' revealed BBC - March 11, 2008
A Nasa space probe measuring the oldest light in the Universe has found that cosmic neutrinos made up 10% of matter shortly after the Big Bang.
Five years of study data also shows that the first stars took over half a billion years to light up the Universe. WMAP launched in 2001 on a mission to measure remnants of light left over from the Big Bang. Scientists say it is collecting a "treasure trove" of information about the Universe's age, make-up and fate. The Wilkinson Microwave Anisotropy probe (WMAP) is mapping the Cosmic Microwave Background (CMB) radiation in the sky. This is the oldest light in the Universe, shifted to microwave wavelengths as the Universe expanded over 13.7 billion years.

What happened before the Big Bang? Physorg - August 3, 2006
Though Einstein's theory of general relativity does an excellent job of describing the universe almost back to its beginning, near the Big Bang matter becomes so dense that relativity breaks down. Beyond that point, we need to apply quantum tools that were not available to Einstein. Now Ashtekar and two of his post-doctoral researchers, Tomasz Pawlowski and Parmpreet Singh, have done just that. Using a theory called loop quantum gravity, they have developed a mathematical model that skates right up to the Big Bang -- and steps through it. On the other side, Ashtekar says, exists another universe with space-time geometry similar to our own, except that instead of expanding, it is shrinking. "In place of a classical Big Bang, there is in fact a quantum Bounce," he says.